The present invention relates generally to a method and apparatus for testing or calibrating magnetic resonance imaging (MRI) systems. More specifically, the invention relates to a device, referred to as a loader, to electrically load RF coils of the MRI system during testing/calibration or operator training.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During MRI operator training or when testing/calibrating the MR system, a suitable substance must be used to simulate the structures and conditions encountered in actual use of the MRI system with a patient. Specifically, properties, or “features,” of human tissue must be simulated to sufficiently represent human tissue such that training in the operation of the MRI system or calibration of the system performance is possible.
One of the “features” of human tissue is its “imagability.” As stated, upon proper excitation, human tissue emits NMR signals. As such, it is possible to receive and reconstruct the emitted signals into an image. Another “feature” of human tissue is electrical conductivity. The electrical conductivity serves to electrically load the RF coils of the MRI system. The load upon the RF coils is directly related to a quality factor (Q) and impedance of the RF coils. As such, load is also related to power required to conduct the scan and the amount of noise introduced into the scan. Therefore, to adequately test and calibrate the MRI system it is necessary to simulate the imagability and electrical load of human tissue.
To simulate human tissue, phantoms have been designed. A single-component phantom is designed to simulate the imagability and electrical load of human tissue with a single apparatus. Typically, in single-component phantoms, multiple fluids are enclosed in separate compartments within the phantom. A first liquid, typically water doped to raise electrical conductivity, is contained within an outer compartment. Salt is commonly used to modify the conductivity of the water. A second liquid, which is relatively non-conductive but which emits NMR signals upon excitation, is contained in an inner compartment. Therefore, a single device, containing a first fluid to provide loading and a second fluid to provide imagability, is used to test or calibrate the MR system.
However, to provide a proper simulation of human tissue it is advantageous that the phantom be comparable in size to the area of human tissue that the phantom is simulating. Therefore, it is necessary to produce relatively large phantoms. However, as the phantom is enlarged, the weight of the phantom can become overly cumbersome because the phantom is filled with two liquids.
To overcome this problem, the single-component phantom is divided into two separate devices. A first phantom, typically referred to as a “loader,” is used to enclose the first, electrically conductive, liquid and a second phantom is used to enclose the second, imagable, liquid. The phantom is designed to be positioned within the loader. As such, the loader and phantom, in combination, serve to provide the electrical conductivity and imagability provided in the single-piece phantoms. The loader provides an electrical load for the RF coils and the phantom provides an imagable volume. When separated, the loader and phantom are considerably more manageable in size and weight than the singe-piece loader. Nevertheless, the loader and phantom are still quite difficult to manage and maneuver due to the liquid contained in each. Furthermore, the liquid-filled loader/phantom combinations are prone to leakage.
A second drawback to the use of either single-piece phantoms or the loader and phantom combination presents itself at high magnetic fields. Above 1.0 T, the large dielectric constant of water (roughly 80) can cause standing wave effects in the water. These standing waves cause a non-uniform RF field inside the phantom even though the RF field applied by the RF coils is uniform. The standing waves, though unrelated to RF coil loading, introduce distortion to the image of the phantom. This deterioration in image quality hinders proper testing or calibration of the imaging system.
Previous attempts at solving this problem include the use of silicone oil, which has a dielectric constant of roughly 5, to replace the water in the phantom and/or loader. However, silicone oil is costly to effectively dope. To achieve the desired electrical conductivity, high concentrations of costly silver powder are required. However, this solution requires the use of a liquid, which can have leakage concerns.
It would therefore be desirable to have a loader with a low dielectric constant and a relatively high electrical conductivity that is effective at high magnetic fields. It would also be desirable to limit the size and weight of the loader to facilitate placement and removal of the loader in the RF coils. Furthermore, it would be advantageous for the loader to be free of liquid, thus, reducing the cost of production and maintenance, and eliminating leakage.